Exploitation has been the economic backbone of every civilization from earliest times to the present day. Each civilization has exploited and used up its natural resources. In every civilization individuals, groups, classes and sometimes castes have exploited or used up fellow humans and fellow creatures to suit their own purposes and advance their own interests. Abraham Lincoln gave a classical definition of human exploitation in a simple sentence: "It is the principal that says you work and toil and earn bread and I will eat it." Exploitation of nature and of fellow beings by man began long before written history. During periods of civilization, and notably in present-day civilization, exploitation has determined social relationships. It has also become one of the pillars of every civilized community. Civilized peoples use up natural resources as a matter of course. The more advanced technically have stripped their environments of replaceable and irreplaceable resources. They have also perfected techniques for using the productive power of their fellow creatures. One way to do this is by owning the body. Another way is ownership of land, capital and consumer goods which enable the owner to live without labor on the products resulting from the labor of others. Owners of property and wealth receive an income because they are owners. They may be very young or very old, able-bodied or helpless. Their livelihood comes to them not because of anything they do, but because of the property titles which they own. The owner of land may collect rent. The owner of capital may collect interest. The owner of an enterprise may collect profits. Each lives by owning. Workers produce goods and services. They are paid an income proportioned to their production. Owners of land, capital and consumer goods are paid incomes proportioned to their ownership. Workers work for a living. Owners live by ownership, chiefly of land and the implements of production. Owners of property frequently are rich. Workers, by comparison, are poor. The line separating owners from workers also separates riches from poverty. Income from services rendered, from work, is earned income. Income from property ownership, by contrast, is unearned income. The relation between earned and unearned income is not confined to one generation. Under laws passed by the owners and their retainers the owners of private property may give or bequeath this property to their descendants. In the course of time a community is divided between workers who are poor and owners who are rich. Since the rich need not work in order to live, they and those associated with them may live on the unearned income derived from property ownership. In a word, they may become parasitic.